CONDITIONS OF EXISTENCE 51 



algae similar to those among the rocky shores of a lake. These are 

 of importance to animals. Sluggish streams have rooted aquatic 

 vegetation. 



The vegetation is used as breeding places. Eggs are stuck into 

 plant tissues by the predaceous diving beetles (Dytiscidae) and by 

 the water scorpions (Ranaira). Eggs are attached to plants by 

 the electric Ught bug (Belostomidae), back swimmers, may-flies, 

 caddis-flies, water scavengers (Hydrophilidse), long horned leaf 

 beetles (Donacia), snails, and many fishes (Umbra, and probably 

 Air amis). Young animals are often dependent upon plants for 

 shelter, to escape from enemies, etc. Many insects must come 

 to the surface for oxygen. The most important of these are the 

 Dytiscidae (adults and larvae), the HydrophiUdae (adults and larvae), 

 the back swimmers, Zaitha, Belostoma, Donacia, snails, Ranatra, 

 and HaUplidae. Some, for example Zaitha and dragon-fly nymphs, 

 He in the vegetation and wait for their prey. 



Different kinds of vegetation have different values for animals. 

 The bulrush is barren for the following reasons: (i) hardness 

 makes it a bad place for eggs; (2) there are no cUnging places; 

 (3) there is little shade; (4) it gives a high temperature in summer; 

 (5) there is no great addition of oxygen by vegetation; (6) it 

 does not afford a suitable place for securing food. Equisetum is 

 unfavorable for similar reasons. Elodea is excellent; Myriophyl- 

 lum, good; water-lilies and Char a, only fair. 



Animal Communities 



Plants and animals select their habitats through physiological 

 characters. Sessile plants and animals have disseminules which 

 usually come to rest in a great variety of conditions and grow to 

 maturity only in those conditions that are suitable to stimulate 

 development. The physiological character of the reproductive 

 bodies and external conditions are responsible for the distribution. 



Animals select their environments by one of three methods: 

 (i) by wide dissemination of reproductive bodies and selective 

 survival, (2) by turning back when the environment in which they 

 move about is found to change, and (3) by selection after trial in 

 connection with migration. 



